In an X-Y addressable imager, the picture elements, called pixels, are arrayed in rows and columns. Charge integrated by photodiodes in respective pixels is read out by scanning all pixels along a row in sequence and row by row. For convenience of description of electronic circuits associated with addressing pixels and for sensing pixel charge, the row circuits have come to be called "horizontal" circuits and column circuits have come to be called "vertical" circuits; and in some cases signals in such circuits are similarly distinguished. This even though there is no implication of physical orientation other than the row and column association. Accordingly, the distinguishing horizontal and vertical qualifiers are employed herein.
Optical imagers with two-transistor pixels and vertical and horizontal scan registers for addressing respective transistors of the pixels are known in the art. One example is to be found in an S. Nishizawa et al. paper "A New, Transversal Signal Line (TSL) Type Solid State Imager," SPSE 26th Fall Symposium, October, 1986, pages 42-47; and it relates to an MOS-type color imager in which each major pixel has four photodiode pixels (one each for the colors white, green, yellow, and blue). Separate horizontal signal lines (HSL) collect pixel output signals for pixels of each color along a row of pixels, and a separate vertical signal line (VSL) for each color is coupled through source-drain paths of switching transistors to collect signals from all HSLs of its color. A reset pulse is applied to each HSL at the onset of its scanning period.
Imagers are usually employed in cooperation with an optical system upon which they rely for controlling the amount of light reflected from a scene being imaged, and which is permitted to be incident upon an image detecting pixel array in accordance with the particular array characteristics. Compared to the time duration of one of plural successive frames of scene information, the time required to adjust the optical system for any particular scene is relatively long. In some applications, it would be useful to be able to adjust effectively the projected light level within, e.g., a frame time.
Also, in some applications it is useful to be able to execute subframe imaging quickly to select a portion of a frame for special processing and to increase the frame rate of the subframe images (e.g., to improve the performance of target tracking systems). However, mechanical systems for accomplishing that end are awkward and time consuming, and do not provide for increased frame rates due to limitations in the read out rate of the imager. Electrical systems for the purpose are expensive because they require extended buffer memories to reform the image data.